Turgor‐sensitive sucrose and amino acid transport into developing seeds of <i>Pisum sativum</i>. Effect of a high sucrose or mannitol concentration in experiments with empty ovules

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of "C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl2. Unloading was stimulated by 15 millimolar ethyleneglycol-bis(6-aminoethyl ether)-N,N,N',N'-tetraacetic acid which partially reversed the inhibitory effects of Ca21. Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

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confidence: 51%

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confidence: 89%

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confidence: 89%

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Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Porter¹,

Knievel²,

Shannon³

1985

“…The rate of photosynthate "unloading" from seed coats ofPhaseolus vulgaris depends on cell turgor potential (17). In developing seeds from Vicia faba and Pisum sativum, the release of sucrose and of amino acids from the seed coat is sensitive to the osmolality of the bathing solution while phosphate release is unaffected by these solutions (23,24). However, the rate of unloading in seed coats of developing soybean seeds 'Supported by the Centre National de la Recherche Scientifique (UA 574).…”

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confidence: 99%

Osmotic Dependence of the Transmembrane Potential Difference of Broadbean Mesocarp Cells

Li¹,

Delrot²

1987

ABSTRACIPod walls of broadbean (Viciafaba L. cv Aguadulce) were harvested at the import (54) at the transition (S2) or at the export (S3) phase for assimilate transport. Measurements of the transmembrane potential difference (PD) of mesocarp cells were made under various osmotic conditions. Internal osmotic potentials and cell turgor were calculated from osmolality measurements of cell saps recovered by freeze-thawing, after correction for the contribution of the free-space solution. Changes in the mannitol concentration of the medium altered the PD within a few minutes, and new stable values of PD were reached within 20 minutes after the osmotic change. With mannitol as the osmoticum, the most negative PD was measured at an external osmotic potential of -0.70 megapascls (MN) for SI and S2, while the most negative was at -0.40 MPa for S3. Ethylene glycol, a permeant osmoticum, had little effect on PD, showing that the PD was sensitive to turgor, not to solute potential per se. For SI and S2, the PD was less negative for turgor potentials lower than 0.1 MPa or greater than 03 MPa. S3 samples exhibited a different turgor dependence, with a sharp optimum of the negativity of the PD at 03 MPa. The data are consistent with the proposal that the proton pump acts as a transducer of the osmotic conditions. They show that the osmotic sensitivity of the PD of mesocarp cells of broadbean changes with the stage of development of the pod. Transport processes at the cell level in the sources and in the sinks of a plant depend on the osmotic environment (2-3, 5, 6, 12, 20, 23, 24). The proton-pumping activity of leaf tissues, as measured by the rate ofacidification oftheir incubation medium, is influenced by the osmotic potential ofthis medium (3,19,25). The uptake of exogenous sugars and amino acids by the leaf is also affected by the concentration and the nature of the osmoticum (2,8,9). Evidence has been given that, in leaf tissues of Phaseolus coccineus, the osmotic conditions altered sucrose uptake, and more particularly its saturable component, via changes in cell turgor (1). Several data suggest that the rate of assimilate transport may also be controlled by the osmotic environment in sink tissues. The rate of photosynthate "unloading" from seed coats ofPhaseolus vulgaris depends on cell turgor potential (17). In developing seeds from Vicia faba and Pisum sativum, the release of sucrose and of amino acids from the seed coat is sensitive to the osmolality of the bathing solution while phosphate release is unaffected by these solutions (23,24). However, the rate of unloading in seed coats of developing soybean seeds 'Supported by the Centre National de la Recherche Scientifique (UA 574).2Supported by a grant from the People's Republic of China.is not affected by mannitol concentrations up to 500 mm (17). In sugar beet taproot tissue, the saturable component of sucrose uptake is sensitive to turgor (25).At the whole plant level, according to the mass-flow model (14), long distance transport also depends on the establishme...

“…While the magnitude of the effect of these inhibitors on a-amino acid unloading was somewhat different compared to that on sugar unloading, the similarity of inhibition patterns suggests a common unloading mechanism. Observations on amino acid and sugar unloading from pea and broad bean seed coats also indicate similar unloading mechanisms in these species (24,25,28). Buffer-treated control samples from the experiment reported in Table VI produced peaks corresponding to 13 different amino acids (Fig.…”

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confidence: 83%

Assimilate Unloading from Maize (Zea mays L.) Pedicel Tissues

Porter¹,

Knievel²,

Shannon³

1987

ABSTRACrSugar, amino acid, and "C-assimilate release from attached maize (Zea mays L.) pedicels was studied following treatment with several chemical inhibitors. In the absence of these agents, sugar release was nearly linear over a 7-hour period. At least 13 amino acids were released with glutamine comprising over 30% of the total. Release was not affected by potassium concentration, 10-minute pretreatments with p-chloromercuribenzene sulfonic acid (PCMBS) or dithiothreitol, and low concentrations of CaC12. Three hours or more exposure to PCMBS, dinitrophenol, N-ethylmaleimide, or 2,4,6-trinitrobenzene sulfonic acid strongly inhibited "C-assimilate, sugar, and amino acid release from the pedicel. These treatments also reduced 'C-assimilate movement into the kernel bases.It is, therefore, likely that reduced unloadings caused by these relatively long-term exposures to chemical inhibitors, was related to reduced translocation of assimilates into treated kernels. Whether this effect is due to disruption of kernel metabolism and sieve element function or reduced assimilate unloading and subsequent accumulation of unlabeled assimilates within the pedicel tissues cannot be determined at this time.Assimilate unloading processes are difficult to understand because unloading mechanisms apparently differ among crop species and among the various sink tissues within a plant (21). In growth sinks, such as root tips and expanding leaves, there is evidence that unloading is via the symplast (5, 19). Within stem tissues, unloading occurs into the apoplast from which assimilates may be taken up for storage or reloaded into the phloem (7,11). In reproductive sinks, such as legume seeds, unloading occurs from the maternal seed coat tissue into the apoplast which surrounds the developing embryo (21). Assimilates must then be taken up into the embryo. Since no vascular or plasmodesmatal connections exist between the maternal tissues and the developing endosperm cells in maize, a similar assimilate pathway to that of legumes must be followed in maize kernels (3,20). We conclude from available evidence (3,12, 15,16,20) PA. Plant culture and experimental conditions for these greenhouse studies have been described (15). Plants which were 21 or 22 d postpollination were utilized for all experiments. Kernels from these plants were in the linear phase of dry matter accumulation and had attained less than 50% oftheir final dry weight. Kernel water content ranged from 55 to 65% by weight. The experiments reported herein were initiated between 9:00 and